It is well known that the frequencies of even the best available crystal oscillators change with time, i.e., all crystal oscillators manifest the phenomenon known as "aging". Also, when irradiated with ionizing radiation (e.g., X-rays, .gamma.-rays, electrons), crystal oscillators change frequency. The amount of radiation induced frequency change, especially at high doses, is known to depend on the purity of the quartz material. The aging of crystal oscillators is generally attributed to phenomena such as mass transfer due to contamination and stress relief.
In order to obtain low aging in crystal oscillators, the "recipe" typically consists of ultraclean processing of the resonator including a high temperature vacuum bake prior to sealing, a mounting structure that is not subject to stress relief, gold electrodes, and a Pierce-type oscillator circuit with automatic gain control and constructed of components selected for high stability. In order to obtain maximum resistance to radiation induced frequency shifts, the "recipe" consists primarily of using swept cultured quartz and oscillator components that are relatively immune to radiation induced changes. "Sweeping" involves heating the quartz, typically to about 500.degree. C., while applying an electric field along the Z crystallographic direction. The strength of the field is typically 1000 V per cm.
In the oscillator circuitry, there is nearly always a small DC voltage applied to the crystal unit due to the bias voltages applied to the transistors in the oscillator circuitry. In the past, this DC voltage was thought to be inconsequential. Also in the past, sweeping to any significant degree was thought to occur only at high temperatures and electric fields.
It has now been found that when a small voltage is applied to a resonator at the normal operating temperatures of oscillators, the aging rate increases significantly. The increase in initial aging rate is significantly higher for resonators made of natural quartz than for those made of the much higher purity swept cultured quartz. It has also been found that when a resonator is irradiated with a pulse of ionizing radiation, the frequency shifts and the conductivity of the quartz plate increase significantly. Immediately after the pulse, conductivity increases as high as a factor of 10.sup.11 have been observed. The conductivity increase is generally attributed to the liberation of alkali impurity ions in the quartz. The higher the alkali impurity level, the higher the conductivity increase. The radiation induced frequency shifts are also known to be associated with the presence of alkali impurities in the quartz.